Energy Requirements of Thermolytic Solutes in Osmotically Driven Membrane Processes

Reverse osmosis (RO) exists as the primary technology used to treat brackish and sea water. However, RO requires large amounts of electrical energy to overcome the natural osmotic pressure of the feed solution. Fortunately, forward osmosis (FO) is a water treatment option that recovers water from feed solutions via a draw solute introduction. Tertiary amines were studied as switchable polarity solvents (SPS) for use in a FO treatment system and can modulate polarity upon reaction with carbon dioxide. The switch reaction of dimethylcyclohexylamine (DMCHA) was studied and samples were analyzed using freezing point osmometry to determine the equilibrium constant and subsequent enthalpies and entropies of reaction. The switch reaction of cyclohexylpiperidine (CHP) was studied earlier and the data was compared with what was generated for DMCHA. At 30°C and 100% CO₂, DMCHA achieved a maximum salt weight percentage of 60.5% while CHP achieved a 55.8% maximum loading. van’t Hoff plots were then generated for each amine. The van’t Hoff behavior of DMCHA interestingly includes two distinct K_eq regions which suggests the amine undergoes two different reactions at different temperature ranges. Understanding the thermodynamics of the switch reaction is important to optimizing the SPS FO water treatment system.